The last time these thrusters were active was in November 1980, when the probe zipped by Saturn.
NASA made the decision to activate the disused thrusters because the thrusters they had been using to adjust the spacecraft's antenna weren't functioning well anymore. In a blog post, the agency explained that Voyager 1's main attitude control thrusters had been degrading, making it hard to reorient the spacecraft so that its antenna points back towards Earth.
On top of the the backups sitting idle for so long, while being 13 billion miles from Earth, there was a third problem: the backups weren't used for "attitude control".
On Tuesday, Voyager engineers sent a command to fire the four "trajectory correction maneuver (TCM)" thrusters and it took 19 hours and 35 minutes for the test results to reach an antenna in Goldstone, California.
"The Voyager flight team dug up decades-old data and examined the software that was coded in an outdated assembler language, to make sure we could safely test the thrusters".
In recent decades, Voyager had been relying on its primary thrusters to keep the spacecraft properly oriented so that it can maintain a communications link with Earth.
After 37 years of disuse, a set of thrusters aboard Voyager 1 activated on Wednesday, firing up humanity's farthest-flung spacecraft and hopefully giving it a longer life than it had before.
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It is expected that in the year 40,272, Voyager 1 will come within 1.7 light years of an obscure star in the constellation Ursa Minor (the Little Bear or Little Dipper) and in about 40,000 years, Voyager 2 will come within about 1.7 light years of a star called Ross 248, a small star in the constellation of Andromeda. But these attitude control thrusters have been degrading over time, requiring more and more energy each time they've been used.
"The mood was one of relief, joy and incredulity after witnessing these well-rested thrusters pick up the baton as if no time had passed at all".
The plan going forward is to switch to the TCM thrusters in January, it said.
The Voyager spacecraft were built by JPL, which continues to operate both.
In 2014, mission managers started noticing that some of Voyager 1's still in service thrusters - called attitude control thrusters - weren't working in top form, NASA said. Now, the Voyager team is able to use a set of four backup thrusters, dormant since 1980.
The thrusters worked just fine. With this example before them, NASA laid a more ambitious message aboard Voyager 1 and 2, a kind of time capsule, meant to relate a story of our world to aliens.